3D MHD Model Research Articles

Improved near optimal angular quadratures for polarised radiative transfer in 3D MHD models

Accurate angular quadratures are crucial for the numerical solution of three-dimensional (3D) radiative transfer problems, especially when the spectral line polarisation produced by the scattering of anisotropic radiation is included. There are two requirements for obtaining an optimal quadrature and they are difficult to satisfy simultaneously: high accuracy and short computing time. By imposing certain symmetries, we were recently able to derive a set of near optimal angular quadratures. Here, we extend our previous investigation by considering other symmetries. Moreover, we test the performance of our new quadratures by numerically solving a radiative transfer problem of resonance line polarisation in a 3D model of the solar atmosphere resulting from a magneto-hydrodynamical simulation. The new angular quadratures derived here outperform the previous ones in terms of the number of rays needed to achieve any given accuracy.

The Atmospheric Wind of Hot Exoplanets and its Manifestations in Observations: from Energy Estimates to 3D MHD Models

The studies of close-orbit gaseous exoplanets—hot Jupiters and warm Neptunes—are reviewed. Transit spectral observations of the Hubble Space Telescope stimulated the numerical modeling of these objects. The supersonic outflow of the upper atmosphere of hot exoplanets and its interaction with the stellar plasma flow is a newly-discovered phenomenon in plasma physics and astrophysics that pools photo- and plasma-chemistry, collision-induced gas-dynamics, and collisionless space plasma. Starting from 2003, the modeling of physical processes in this field has considerably developed from simple quasi-empiric analytical formulas to global three-dimensional magnetic hydrodynamic (3D MHD) codes. The attained level makes it possible to interpret the in-transit measurements in absorption lines of different elements and estimate the parameters of both the planetary atmospheres and the stellar plasma wind.

The Impact of Eruptions from Young Stars on Environments of Rocky Exoplanets

Kepler and TESS missions have discovered over 4500 extra solar (exoplanets) around F, G, K and M dwarfs. They also revealed frequent superflares on planet hosting stars, providing a mechanism by which host stars may have profound effects on the physical and chemical evolution of exoplanetary atmospheres. While we can only infer the course of the Sun’s early evolution and how it might have affected the early evolution of the Earth, possibly setting the stage for the origin of life, the observation of planets around sun-like stars allows us to directly observe events which likely took place in our own solar system. A major question this leads to is: what effects do extreme energy fluxes from eruptive events during evolution of G-K planet hosts have on prebiotic chemistry and primitive life forms on primitive planets? To address this question, I will describe recent observations of young solar-like stars as inputs for our 3D MHD models of the corona, the wind and transient events (flares, coronal mass ejections and solar energetic particle events) and discuss their impact on atmospheric erosion and chemistry of our planet. I will then use these constrained energy fluxes to describe our recent atmospheric chemistry models impacted by energetic particles from the young Sun and formation and precipitation of biologically relevant molecules. I will then highlight our results of laboratory experiments of proton irradiation of mildly reduced gas mixtures and their implications to the climate, prebiotic chemistry and the rise of habitability on early Earth and young exoplanets.

Coronal Heating by MHD Waves

The heating of the solar chromosphere and corona to the observed high temperatures, imply the presence of ongoing heating that balances the strong radiative and thermal conduction losses expected in the solar atmosphere. It has been theorized for decades that the required heating mechanisms of the chromospheric and coronal parts of the active regions, quiet-Sun, and coronal holes are associated with the solar magnetic fields. However, the exact physical process that transport and dissipate the magnetic energy which ultimately leads to the solar plasma heating are not yet fully understood. The current understanding of coronal heating relies on two main mechanism: reconnection and MHD waves that may have various degrees of importance in different coronal regions. In this review we focus on recent advances in our understanding of MHD wave heating mechanisms. First, we focus on giving an overview of observational results, where we show that different wave modes have been discovered in the corona in the last decade, many of which are associated with a significant energy flux, either generated in situ or pumped from the lower solar atmosphere. Afterwards, we summarise the recent findings of numerical modelling of waves, motivated by the observational results. Despite the advances, only 3D MHD models with Alfv\'en wave heating in an unstructured corona can explain the observed coronal temperatures compatible with the quiet Sun, while 3D MHD wave heating models including cross-field density structuring are not yet able to account for the heating of coronal loops in active regions to their observed temperature.

Simulations of Magnetised Stellar-Wind Bubbles

Initial results are presented from 3D MHD modelling of stellar-wind bubbles around O stars moving supersonically through the ISM. We describe algorithm updates that enable high-resolution 3D MHD simulations at reasonable computational cost. We apply the methods to the simulation of the astrosphere of a rotating massive star moving with 30km s−1 through the diffuse interstellar medium, for two different stellar magnetic field strengths, 10G and 100G. Features in the flow are described and compared with similar models for the Heliosphere. The shocked interstellar medium becomes asymmetric with the inclusion of a magnetic field misaligned with the star’s direction of motion, with observable consequences. When the Alfv´enic Mach number of the wind is ≤ 10 then the stellar magnetic field begins to affect the structure of the wind bubble and features related to the magnetic axis of the star become visible at parsec scales. Prospects for predicting and measuring non-thermal radiation are discussed.

Magnetorotational core-collapse supernovae: the impact of the magnetic field’s structure

AbstractThe combination of strong magnetic fields and fast rotation is often invoked as a characteristic of the central engine for outstanding sources such as GRBs, hypernovae, and superluminous supernovae. However, the actual properties of the magnetic field during the collapse of the stellar progenitor are very uncertain, since they depend on the evolution of the star and can be affected by complex dynamo processes occurring in the central proto-neutron star. Using 3D relativistic MHD models we show that higher-order multipolar fields can lead to the onset of a supernova, although they tend to produce less energetic explosions and less collimated outflows. Quadrupolar fields efficiently extract angular momentum from the central core, but the rotational energy is partly stored in the equatorial regions, rather than powering up the polar outflows. Finally, our results show a strong magnetic quenching of the hydrodynamic non-axisymmetric instabilities that are associated to the emission of GWs.

Shattered pellet penetration in low and high energy plasmas on DIII-D

Shattered pellet injection (SPI) has been adopted as the baseline disruption mitigation system for ITER, as the radiative payload penetration into DIII-D plasmas from SPI is superior to those using the massive gas injection (MGI) method. Because of the substantial differences in the energy content of ITER plasma and those in present experiments, reliable 3D MHD modeling, benchmarked against present experiments is needed to project to ITER plasmas. In support of these needs, the depth of SPI fragment penetration in DIII-D plasmas was investigated by injecting SPI into two discharges with vastly different energy content and pedestal height. 400 Torr-L pure Ne fragmented pellets at a velocity of about 200 m s−1 were injected into a 0.2 MJ L-mode discharge and a 2 MJ super H-mode discharge. Results show deep penetration of SPI fragments into low-energy plasmas in DIII-D. SPI fragment penetration is reduced as the plasma energy content increases, with some discharges exhibiting penetration that is confined to the outer regions of the plasma. The injected SPI fragments are also spread out over a distance of about 20 cm, which results in some fragments arriving near the end of or after the thermal quench is over.

Modeling inner boundary values at 18 solar radii during solar quiet time for global three-dimensional time-dependent magnetohydrodynamic numerical simulation

We develop an empirical model of the solar wind parameters at the inner boundary (18 solar radii, Rs) of the heliosphere that can be used in our global, three-dimensional (3D) magnetohydrodynamic (MHD) model (G3DMHD) or other equivalent ones. The model takes solar magnetic field maps at 2.5 R, which is based on the Potential Field Source Surface, PFSS model and interpolates the solar wind plasma and field out to 18 Rs using the algorithm of Wang and Sheeley (1990). A formula (V18Rs = V1 + V2fsα) is used to calculate the solar wind speed at 18 Rs, where V1 is in a range of 150–350 km s−1, V2 is in the range of 250–500 km s−1, and “fs” is the magnetic flux expansion factor derived from the Wang and Sheeley (WS) algorithm at 2.5 R. To estimate the solar wind density and temperature at 18 Rs, we assume an incompressible solar wind and a constant total pressure. The three free parameters are obtained by adjusting simulation results to match in-situ observations (Wind) for more than 54 combinations of V1, V2 and α during a quiet solar wind interval, i.e., the Carrington Rotation (CR) 2082. We found that VBF = (200 ± 50) + (400 ± 100) fs−0.4 km/s is a good formula for the quiet solar wind period. The formula was also good to use for the other quiet solar periods. Comparing results between WSA (Arge et al. 2000, 2004) and our model (WSW-3DMHD), we find the following: i) The results of using VBF with the full rotation (FR) data as input to drive the 3DMHD model is better than the results of WSA using FR, or daily updated. . ii) The WSA model using the modified daily updated 4-day-advanced solar wind speed predictions is slightly better than that for WSW-3DMHD. iii) The results of using VBF as input to drive the 3DMHD model is much better than the using the WSA formula with an extra parameter for the angular width (θb) from the nearest coronal hole. The present study puts in doubt in the usefulness of θb for these purposes.

Assessment of CESE-HLLD ambient solar wind model results using multipoint observation

For a three-dimensional magnetohydrodynamics solar wind model, it is necessary to carry out assessment studies to reveal its ability and limitation. In this paper, the ambient solar wind results of year 2008 generated by the CESE-HLLD 3D MHD model are compared with multipoint in-situ measurements during the late declining phase of solar cycle 23. The near-ecliptic results are assessed both quantitatively and qualitatively by comparing with in-situ data obtained at the L1 point and by the twin STEREO spacecraft. The assessment reveals the model’s ability in reproducing the time series and statistical characteristics of solar wind parameters, and in catching the change of interplanetary magnetic field polarity and the occurrence of the stream interaction regions. We find that the two-stream structure observed near the ecliptic plane is reproduced, but the differences among observations at L1 and the twin STEREO spacecraft are not caught by the model. The latitudinal variation of the results is assessed by comparing with the Ulysses observation. The characters of variation in different latitudinal ranges are duplicated by the model, but biases of the results are seen, and the boundary layers between fast and slow solar wind are sometimes thicker than observation.

3D MHD Modeling of the Impact of Subsurface Stratification on the Solar Dynamo

Abstract Various models of solar subsurface stratification are tested in the global EULAG-MHD solver to simulate diverse regimes of near-surface convective transport. Sub- and superadiabacity are altered at the surface of the model (r > 0.95R ⊙) to either suppress or enhance convective flow speeds in an effort to investigate the impact of the near-surface layer on global dynamics. A major consequence of increasing surface convection rates appears to be a significant alteration of the distribution of angular momentum, especially below the tachocline where the rotational frequency predominantly increases at higher latitudes. These hydrodynamic changes correspond to large shifts in the development of the current helicity in this stable layer (r < 0.72R ⊙), significantly altering its impact on the generation of poloidal and toroidal fields at the tachocline and below, acting as a major contributor toward transitions in the dynamo cycle. The enhanced near-surface flow speed manifests in a global shift of the toroidal field (B ϕ ) in the butterfly diagram, from a north–south symmetric pattern to a staggered antisymmetric emergence.

Cosmic ray interactions in the solar atmosphere

ABSTRACT High-energy particles enter the solar atmosphere from Galactic or solar coronal sources, and produce ‘albedo’ emission from the quiet Sun that is now observable across a wide range of photon energies. The interaction of high-energy particles in a stellar atmosphere depends essentially upon the joint variation of the magnetic field and plasma density, which heretofore has been characterized parametrically as P ∝ Bα with P the gas pressure and B the magnitude of the magnetic field. We re-examine that parametrization by using a self-consistent 3D MHD model (Bifrost) and show that this relationship tends to P ∝ B3.5 ± 0.1 based on the visible portions of the sample of open-field flux tubes in such a model, but with large variations from point to point. This scatter corresponds to the strong meandering of the open-field flux tubes in the lower atmosphere, which will have a strong effect on the prediction of the emission anisotropy (limb brightening). The simulations show that much of the open flux in coronal holes originates in weak-field regions within the granular pattern of the convective motions seen in the simulations.

On the origin of jet-like features in bow shock pulsar wind nebulae

ABSTRACT Bow shock pulsar wind nebulae are a large class of non-thermal synchrotron sources associated to old pulsars that have emerged from their parent supernova remnant and are directly interacting with the interstellar medium. Within this class a few objects show extended X-ray features, generally referred as ‘jets’, that defies all the expectations from the canonical MHD models, being strongly misaligned respect to the pulsar direction of motion. It has been suggested that these jets might originate from high energy particles that escape from the system. Here we investigate this possibility, computing particle trajectories on top of a 3D relativistic MHD model of the flow and magnetic field structure, and we show not only that beamed escape is possible, but that it can easily be asymmetric and charge separated, which as we will discuss are important aspects to explain known objects.

Two Types of Confined Solar Flares

Abstract With the aim of understanding the physical mechanisms of confined flares, we selected 18 confined flares during 2011–2017, and first classified them into two types based on their different dynamic properties and magnetic configurations. “Type I” confined flares are characterized by slipping reconnection, strong shear, and a stable filament. “Type II” flares have almost no slipping reconnection, and have a configuration in potential state after the flare. A filament erupts but is confined by a strong strapping field. “Type II” flares could be explained by 2D MHD models, while “type I” flares need 3D MHD models. Seven of 18 confined flares (∼39%) belong to “type I” and 11 (∼61%) are “type II.” The post-flare loops (PFLs) of “type I” flares have a stronger non-potentiality, but the PFLs in “type II” flares are weakly sheared. All the “type I” flares exhibit ribbon elongations parallel to the polarity inversion line (PIL) at speeds of several tens of km s−1. Only a small proportion of “type II” flares show ribbon elongations along the PIL. We suggest that different magnetic topologies and reconnection scenarios dictate the distinct properties for the two types of flares. Slipping magnetic reconnections between multiple magnetic systems result in “type I” flares. For “type II” flares, magnetic reconnections occur in antiparallel magnetic fields underlying the erupting filament. Our study shows that “type I” flares account for more than one third of all the large confined flares, and should not be neglected in further studies.

Realistic 3D MHD modeling of self-organized magnetic structuring of the solar corona

AbstractThe dynamics of solar magnetoconvection spans a wide range of spatial and temporal scales and extends from the interior to the corona. Using 3D radiative MHD simulations, we investigate the complex interactions that drive various phenomena observed on the solar surface, in the low atmosphere, and in the corona. We present results of our recent simulations of coronal dynamics driven by underlying magnetoconvection and atmospheric processes, using the 3D radiative MHD code StellarBox (Wray et al. 2018). In particular, we focus on the evolution of thermodynamic properties and energy exchange across the different layers from the solar interior to the corona.

MHD modeling of rotating arc under restrike mode in ‘Kvaerner-type’ torch: II. Dynamics and stability at 20 bar pressure

This paper focuses on the 3D MHD modeling of a very high-pressure (20 bar) arc plasma at industrial level; the torch is a ‘Kvaerner-type’ one, made of two concentric graphite electrodes and where spots are displaced with an external magnetic field. Different phenomenologies in arc movement were observed, ranging from a gliding arc on electrodes surfaces to hopping arc foots and notably impacting electrodes erosion and the overall efficiency in the plasma process. Besides those dynamics which are indebted to torch configuration, current and flow rates, arc stability is also investigated with a theoretical model based on an energy principle. This research work provides new insight on the dynamics and stability of the arc inside the Kvaerner-type torch along with an unprecedented understanding of the very high pressure arc under concentric-electrodes-configuration, highlighting strong deformations due to pressure-driven instabilities for the high power arc.

MHD modeling of rotating arc under restrike mode in ‘Kvaerner-type’ torch: part I. Dynamics at 1 bar pressure

This paper focuses on the 3D MHD modeling of an arc plasma at industrial level; the torch is a ‘Kvaerner-type’ one, made of two concentric graphite electrodes and where spots are displaced with an external magnetic field. Different phenomenologies in arc movement were observed, ranging from a gliding arc on electrodes surfaces to hopping arc foots, agreeing with previous experimental works at atmospheric pressure. These dynamics have non-negligible consequences in the plasma process: the hopping mode induces more erosion due to a higher heat flux at the electrodes surfaces in comparison to the gliding arc; the latter mode results also in a strong gas swirling inside the torch, thus longer particles residence time is predicted.

3D MHD modeling of the expanding remnant of SN 1987A

Aims. We investigate the role played by a pre-supernova (SN) ambient magnetic field in the dynamics of the expanding remnant of SN 1987A, and the origin and evolution of the radio emission from the remnant, in particular during the interaction of the blast wave with the nebula surrounding the SN. Methods. We modeled the evolution of SN 1987A from the breakout of the shock wave at the stellar surface to the expansion of its remnant through the surrounding nebula using three-dimensional magnetohydrodynamic simulations. The model considers the radiative cooling, the deviations from equilibrium of ionization, the deviation from temperature-equilibration between electrons and ions, and a plausible configuration of the pre-SN ambient magnetic field. We explore the strengths of the pre-SN magnetic field ranging between 1 and 100 μG at the inner edge of the nebula and we assume an average field strength at the stellar surface B0 ≈ 3 kG. From the simulations, we synthesize the thermal X-ray and the non-thermal radio emission and compare the model results with observations. Results. The presence of an ambient magnetic field with strength in the range considered does not change significantly the overall evolution of the remnant. Nevertheless, the magnetic field reduces the erosion and fragmentation of the dense equatorial ring after the impact of the SN blast wave. As a result, the ring survives the passage of the blast, at least during the time covered by the simulations (40 yr). Our model is able to reproduce the morphology and lightcurves of SN 1987A in both X-ray and radio bands. The model reproduces the observed radio emission if the flux originating from the reverse shock is heavily suppressed. In this case, the radio emission originates mostly from the forward shock traveling through the H II region and this may explain why the radio emission seems to be insensitive to the interaction of the blast with the ring. Possible mechanisms for the suppression of emission from the reverse shock are investigated. We find that synchrotron self-absorption and free–free absorption have negligible effects on the emission during the interaction with the nebula. We suggest that the emission from the reverse shock at radio frequencies might be limited by highly magnetized ejecta.

Ion Charge States in a Time-Dependent Wave-Turbulence-Driven Model of the Solar Wind

Ion fractional charge states, measured in situ in the heliosphere, depend on the properties of the plasma in the inner corona. As the ions travel outward in the solar wind and the electron density drops, the charge states remain essentially unaltered or "frozen in". Thus they can provide a powerful constraint on heating models of the corona and acceleration of the solar wind. We have implemented non-equilibrium ionization calculations into a 1D wave-turbulence-driven (WTD) hydrodynamic solar wind model and compared modeled charge states with the Ulysses 1994-5 in situ measurements. We have found that modeled charge state ratios of $C^{6+}/C^{5+}$ and $O^{7+}/O^{6+}$, among others, were too low compared with Ulysses measurements. However, a heuristic reduction of the plasma flow speed has been able to bring the modeled results in line with observations, though other ideas have been proposed to address this discrepancy. We discuss implications of our results and the prospect of including ion charge state calculations into our 3D MHD model of the inner heliosphere.

Quasi-periodic Counter-propagating Fast Magnetosonic Wave Trains from Neighboring Flares: SDO/AIA Observations and 3D MHD Modeling

Abstract Since their discovery by the Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA) in the extreme ultraviolet, rapid (phase speeds of ∼1000 km s−1), quasi-periodic, fast-mode propagating (QFP) wave trains have been observed accompanying many solar flares. They typically propagate in funnel-like structures associated with the expanding magnetic field topology of the active regions (ARs). The waves provide information on the associated flare pulsations and the magnetic structure through coronal seismology (CS). The reported waves usually originate from a single localized source associated with the flare. Here we report the first detection of counter-propagating QFPs associated with two neighboring flares on 2013 May 22, apparently connected by large-scale, trans-equatorial coronal loops. We present the first results of a 3D MHD model of counter-propagating QFPs in an idealized bipolar AR. We investigate the excitation, propagation, nonlinearity, and interaction of the counter-propagating waves for a range of key model parameters, such as the properties of the sources and the background magnetic structure. In addition to QFPs, we also find evidence of trapped fast- (kink) and slow-mode waves associated with the event. We apply CS to determine the magnetic field strength in an oscillating loop during the event. Our model results are in qualitative agreement with the AIA-observed counter-propagating waves and used to identify the various MHD wave modes associated with the observed event, providing insights into their linear and nonlinear interactions. Our observations provide the first direct evidence of counter-propagating fast magnetosonic waves that can potentially lead to turbulent cascade and carry significant energy flux for coronal heating in low-corona magnetic structures.

On the problem of two-tail heliosphere/astrospheres

We consider the effect of the solar/stellar wind collimation towards the solar/stellar wind rotation axis in the heliosheath between the termination shock and the heliopause/astropause. The collimation is due to the magnetic force produced by the toroidal component of the solar/stellar magnetic field. The collimation leads to formation of a two-jet structure and change of topology of the heliopause. Tube-like shape of the heliopause/astropause is formed instead of the commonly accepted sheet-like shape.Three different situations are explored in this paper: (1) the Sun/star is at the rest with respect to local interstellar medium (LISM), (2) the Sun/star moves with respect to fully ionized LISM, (3) the Sun/star moves with respect to partially ionized LISM. 3D non-dissipative MHD model results have shown that the tube-like structure is formed in the first two cases. The thickness of the heliosheath depends on the model parameters strongly. The case of the partially ionized LISM is the most realistic one for the heliosphere. In this case the collimation towards the solar poles is also observed in the results of 3D kinetic-MHD modeling. However, the tube-like structure of the heliopause is not seen in the numerical results. We argue that this is due to charge exchange between proton and H atom components that results in displacement of the stagnation points in further downwind of the Sun. Note also that dissipative effects (e.g. reconnection at the current sheet in the non-stationary heliosphere) or instabilities could destroy the effects of the solar wind collimation.